Lamp flicker is a known problem in high pressure lamps, such as video projection lamps. It is critical to reduce the flicker in order to achieve high performance in projection lamp applications. The following relates to operating a high pressure lamp to reduce flicker.
Lamp flicker in a high pressure lamp is associated with the temperature and physical characteristics of the electrode tip protrusions. Often, a discharge arc from the cathode electrode to the anode electrode is not stable because the origin of the discharge arc is not stable. An electrode that is too cold or hot will produce micro spikes resulting from the electrode discharge arc jumping from one spot on the electrode to a different spot on the electrode. This is perceived by a viewer as flicker. U.S. Pat. No. 5,608,294 (hereafter referred to as Derra et al.) proposes one solution to this problem and discusses the reasons for flicker understood by one of skill in the art of high pressure lamps. Derra et al. discloses an alternating lamp current with a current pulse superimposed on the latter part of the half cycle of the alternating lamp current for operating a high pressure discharge lamp to reduce flicker. The method disclosed by Derra et al. is limited to a lamp current waveform supplying a lamp that includes a current ratio Ri in the range of 0.6 to 2.0 and a duration ratio Rt in the range of 0.05 to 0.15. The current ratio Ri is defined as the mean amplitude of the current pulse to the mean amplitude of the alternating lamp current. The duration ratio Rt is defined as the duration of the current pulse to the duration of a half cycle of the alternating lamp current. U.S. Pat. No. 4,485,434 (hereafter referred to as Beeston et al.) describes a power supply for supplying power to arc lamps. U.S. Pat. No. 5,608,294, Derra et al., and U.S. Pat. No. 4,485,434, Beeston et al. are hereby incorporated by reference.
There is a need for current waveforms, other than those taught in Derra et al., to control lamp flicker in high intensity discharge lamps. Additional waveforms will provide alternatives to the designer of lamp circuits to minimize associated costs to reduce flicker. In addition, there is a need for a device and method to generate alternative waveforms that include the combination of an alternating lamp current and one or more pulses. Finally, there is a need for a method of controlling a high intensity lamp to maintain constant power and reduce flicker by varying Rt and Ri.